The blazing horses of Phaethon

You saw some Geminid meteors last night, I hope? Now maybe you’d like to know what those things were.

When I was asked to do an article about them, I entitled it “Nights of Incandescent Dust,” because that’s what most meteors are. They are small solid bits, from dust up to sand and pebbles and a few larger, which have been orbiting in space and, as they plunge into our atmosphere at up to 70 kilometers a second, are vaporized partly or wholly by the friction, reaching temperatures that cause them to give off light.

The editors changed my title to “Showers of Fire.” Corny, uninformative! But later I learned that my “Incandescent Dust” wasn’t such a fitting title either.

The origin of the Geminids was long a mystery. Which comet did they derive from? Since Giovanni Schiaparelli demonstrated it for the Perseid shower of 1833, we have known that these bits of matter are traveling in the approximate orbits of comets. Comets’ bodies (this part of the theory dates from Fred Whipple in 1950) are clumps of rather crumbly solid stuff mixed with ice. The radiation from the Sun, when a comet passes near enough to it, sublimes some of the ice and thus releases some of the bits of solid, and they slowly drift off into their own orbits. The parent comets have been identified for many of the annual showers: the Perseids are debris from Comet 109P Swift-Tuttle, the Eta Aquarids and Orionids from Halley’s Comet, the Leonids from Comet 55P Tempel-Tuttle, and so on. A sort of widespread cloud of meteors (not easy to discriminate into streams) has been shed from Comet 2P Encke, which has the shortest orbit, only 3.3 years, so that it comes around most often and is so often near the Sun.

But what about the Geminids? They are in a general orbit even shorter than that of Encke’s Comet, looping around and around the Sun in only about one and a half years. But no comet could be found in such an orbit. Surprising that such an abundant and steady stream would not betray its source!


The answer came from IRAS, the Infrared Astronomical Satellite, which was launched in January 1983, worked till November of that year, and still blindly orbits the Earth. The function of IRAS was to survey fixed infrared sources, of which it found 350,000, mostly identified as remote stars and galaxies in various stages of their lives. So how come it caught this fast-moving object? What happened was that a team at Leicester University decided to study IRAS’s rejects. These were the relatively few sources that jumped from place to place between the four times IRAS’s gaze passed over them. Thus they found three asteroids and six comets. One of the asteroids. in images from October 1983, had the preliminary designation 1983 TB. When its orbital elements were published, Fred Whipple pointed out that they were close to those calculated for some photographed Geminid meteors. Here at last was the Geminids’ parent. It got the permanent asteroid number and name 3200 Phaethon.

Unlike other known shower meteors (though like some meteorites that reach the surface of the Earth), the Geminids have traveled to us not from a comet but from an asteroid. This explains their look. They typically, as you may have seen, make long, fairly slow, bright trails; they do not fizz or leave lingering trains. The medium-slowness is because they come in across our orbit from the side rather than from ahead; the steady brightness is because they are incandescing rocks, not dust. Asteroids are flying mountains, more robust lumps than comets (some of them are the cores of comets that have lost their looser outer material).

On its way in, the asteroid passes very close over the December part of Earth’s orbit. Usually Earth is somewhere else, but on 2017 Dec. 10 Phaethon will pass us at about 10 million kilometers (compare the 93 million to the Sun). Will that mean that we pass through a thicker part of the meteor stream? Maybe Alastair McBeath will comment.

Numbers are given to asteroids when their orbits are confirmed; they get names later. Why “Phaethon”? It plunges near to the Sun. It is an Apollo-type asteroid, meaning that its average distance is outside Earth’s but it crosses Earth’s orbit inward. Its orbit is only 1.4 years – much shorter than any comet’s, longer than Earth’s, but, instead of being nearly circular like Earth’s, is very elongated. It soars out well beyond Mars, then dives back to little more than a third of Mercury’s distance from the Sun. At that time its surface may heat to 750 degrees Celsius (1400 Fahrenheit). No wonder it loses some skin to sunburn.

Phaethon (or Phaëthon to show that the vowels are proncounced separately) means “shining.” He was a son of the Sun-god Helios. He begged his father to let him drive the chariot of the Sun for a day; could not control the fiery horses; the Sun swerved disastrously and, in the version elaborated by Ovid, scorched much of Africa to a desert and burned its people black; Zeus had to kill Phaethon with a thunderbolt, and his body fell into the river Eridanus.

In Paris you maybe can still drive the kind of light, fast, huge-wheeled carriage called a phaéton, but be careful.




2 thoughts on “The blazing horses of Phaethon”

  1. Sadly no luck for me with the 2014 Geminids, thanks to cloudy skies on nights closest to the expected maximum. However, plenty of others elsewhere were more fortunate. The International Meteor Organization’s preliminary visual results page shows how the incoming data have stacked up into Zenithal Hourly Rates across the peak so far, at .

    The Geminids indeed seem to follow an orbit remarkably similar to that of 3200 Phaethon, strongly suggesting that object is the parent of the shower, although whether it’s a “true” asteroid or the largely degassed remains of an extinct comet remains open to question. Jure Atanackov’s comments about it on the IMO website in advance of this year’s Geminids mentioned that “recent observations with the STEREO spacecraft have revealed cometary activity near perihelion, so it is also referred to as a “rock comet”. It is hypothesized that the dust release is caused by thermal fracturing of the surface of the comet under intense heating near the Sun.” The technical paper dealing with this was published in The Astronomical Journal in late 2010 by David Jewitt and Jing Li, and is available as a free PDF download here .

    Similarly, while apparent differences between Geminid meteoroids and those of more plausibly “cometary” meteor showers have sometimes been supposed to support this different origin, this too remains uncertain. Jiri Borovicka argued at the IAU Symposium on Near-Earth Objects back in 2006 (No.236) that the nature of the Geminid meteoroids was not evidence for them being asteroidal in nature, for instance. His paper can be downloaded, again as a free PDF, via this direct link from the Cambridge University Press website .

    Of course the strength of the materials one comet is made from has recently been directly examined thanks to the Philae lander on Comet 67P Churyumov–Gerasimenko, which found the comet’s surface surprisingly far harder than expected, suggesting this body at least may not be quite the “dusty snowball” some comet theories have long favoured.

    As for what may happen in 2017, modelling of the Geminid meteoroid stream in space has proven rather tricky overall. Nor are there many specific predictions for what may happen during that “close encounter” scenario as yet. Possibly more will appear over the next couple of years. However, the IMCCE website (Institut de Mécanique Céléste et de Calcul des Ephémérides) does have a prediction for the 2017 Geminids now, via the meteor showers ephemeris generator webpage. The direct link for the 2017 Geminids JPEG diagram is here. This suggests the Earth will probably miss the densest part of the trail – at least as far as this model simulation allows us to tell.

  2. San Francisco’s sky was mostly to fully cloudy last night, and tonight it is starting to rain. No visible Geminids this year. But we badly need the rain, so no complaints.

    Thanks for the explanation, and for letting me know I’ve been mispronouncing Phaethon.

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